Contributor: Stephen D. McLeod
Onchocerciasis (river blindness) is an infection caused by the nematode microfilarial parasite Onchocerca volvulus. This nematode disease involves early skin rash and followed by skin nodules, and extensive eye involvement, including keratitis, anterior uveitis [uveitis - anterior], chorioretinitis, optic neuritis, and optic atrophy. Onchocerciasis is by far the most important helminth disease of the eye. The disease is endemic across equatorial Africa where an estimated 99% of those infected live, and in certain areas of Central and South America. Up to 86 million are at risk of acquiring infection, and an estimated 600,000 people are visually impaired by the disease, with half of them partially or totally blind. In hyperendemic areas, infection rates approach 100%, and over a lifetime, 50% of the population will be blinded by the disease. Onchocerca volvulus is transmitted by bites of infected black simulian flies (family Simuliidae), which breed in swift running streams in parts of tropical West Africa, South America, Guatemala and Mexico. Microfilaria are ingested by the female black fly when she feeds on an infected person. The microfilaria pass out of the digestive system and enter the flight muscles. After a number of molts, infective larva migrate to the proboscis and from thence can be transmitted to the next individual attacked by the blackfly. In the new human host, the microfilaria migrate through subcutaneous tissues. Their distant dissemination and invasion of the eye can occur via the blood stream or by direct invasion along vessels and nerves.